The invention relates to a method for grinding the exterior of a shaft part with rotationally symmetrical sections and end surfaces which have centering bores, as well as to a system composed of a grinding machine and such a shaft part.
In grinding technology, fundamentally two different technologies exist, which also require different grinding machines, and the grinding methods of which differ fundamentally from one another. For one thing, this is grinding between tips, in which a workpiece to be ground is usually held, during the grinding process, between a tip disposed on the workpiece headstock and a tip disposed on the opposite tailstock. The workpiece can be driven to rotate either by means of the friction of the tip on the workpiece headstock side in the center, or by means of separate drivers, or by means of a clamping chuck having equalizing clamping jaws. For such grinding between tips it is necessary, particularly in the case of longer workpieces, which demonstrate a certain flexibility with regard to possible bending, to support these at one or more locations of the workpiece during grinding, by means of bezels, in order to obtain an appropriately good quality of the grinding result.
A second fundamental grinding method is what is called centerless grinding. In centerless grinding, the workpiece is not held between tips; instead, it is put into rotation by means of a grinding disk and a regulation disk that lies opposite to the former, for grinding or during grinding, wherein what is called a support ruler is provided in the grinding gap between the grinding disk and the regulation disk, under the workpiece, on which ruler the workpiece is supported during grinding. Such a centerless grinding method is particularly suitable for grinding the exterior of rotationally symmetrical workpieces or workpieces having rotationally symmetrical sections, which workpieces are provided in great numbers for mass production. The disadvantage in centerless grinding consists in that the relationship with centering no longer exists in the case of tip-free grinding.
In DE 10 2008 045 842 B4, a method and a grinding machine for grinding elongated workpieces are known. With this known grinding machine and this known method, elongated rotationally symmetrical workpieces are ground by means of two opposite grinding disks that engage at the same time, wherein because of their opposite rotation, the workpiece is pressed against a support device disposed laterally in the grinding gap between the grinding disks. In this regard, the workpiece is clamped between tips, and this is possible because the grinding forces of the opposite grinding disks cancel one another out. The workpiece is pressed against the support element by means of the forces that occur during cutting. This is therefore passive support, because not only the grinding disks but also the support element is/are positioned in the same way, so that the forces that act on the workpiece in the tangential direction cancel one another out. The equal positioning of grinding disks and support element allows no reaction of the system to deviations between reference diameter and actual diameter. As a result, limits are set for the precision that can be achieved.
In the online article of the may company, a cylindrical grinding machine for grinding between tips and for tip-free grinding is described. The combination of grinding between tips and centerless grinding is supposed to lead to a significant reduction in the machining time, with a simultaneous increase in quality as compared with grinding only between tips. In the known machine, a predetermined amount of material is ground away during grinding between tips. In order to subsequently be able to grind in centerless manner, clamping of the workpiece between the tips is released, and subsequently, the centerless grinding process is carried out. In known manner, in this regard, the workpiece is driven by the regulation disk and supported on every seat. When the centering tips retract for the purpose of releasing the tips from the workpiece, the workpiece drops to the contact rail disposed in the grinding gap. Even in this regard, it is not ensured that the relationship with centering, which is predetermined by means of clamping between the tips, is maintained or even maintained only within low tolerances, because a change in reference takes place during the grinding process.
Furthermore, in a press release dated Feb. 22, 2006, on the internet, an article regarding a “Kronos I dual” grinding machine is described, with which both centerless grinding and grinding between tips is possible in one machine. In this regard, however, the workpiece is also first ground between the tips, and afterward, the workpiece is released from the clamping between tips, and finishing grinding is carried out by means of centerless grinding. Here, too, the relationship with centering is given up in the transition from grinding between tips to centerless grinding, and therefore precision losses are connected with this, since the aforementioned change in reference during grinding takes place.
The known grinding machine Kronos L dual uses both grinding methods, grinding between tips and centerless grinding, on one machine. In this regard, pre-grinding takes place between tips, during which a clean, cylindrical outer surface is produced on the rotationally symmetrical sections. After completion of pre-grinding between tips, the tips are released, and centerless grinding follows. As a result, a new relationship of what is called the center occurs, specifically by way of the clean, cylindrical surfaces of the rotationally symmetrical sections, which were produced during pre-grinding. Therefore, a certain preservation of the center of the workpiece is achieved, but this is not sufficient for the highest quality demands, because what is called the center is only maintained to the extent that it permits the precision of the clean, cylindrical surfaces of the rotationally symmetrical sections of the workpiece produced during pre-grinding.
Using the known machine, the grinding process is interrupted after pre-grinding, in any case, and after the subsequent release of the tension between the tips, when the tip tension on the workpiece is released. This is then followed by the newly started centerless grinding. This means that aside from the change in the reference longitudinal axis of the workpiece, the grinding time is extended.
On the other hand, the disadvantage consists in that because of the clear technological separation between grinding between tips and centerless grinding, the workpieces are not reliably supported during grinding between tips, neither by the contact ruler nor by the regulation disk that is generally present during centerless grinding. Therefore, difficulties can occur during grinding, particularly in the case of long, thin shafts, which are relatively flexible and unstable.
If, furthermore, particularly in the case of long and thinner workpieces, grinding is to take place between tips, it is usual to provide bezels for additional support. For this purpose, however, it is necessary that corresponding bezel seats must be ground on before the bezels can be set onto the workpiece. Grinding of the bezel seats requires an additional expenditure of time, which is all the greater, the longer the workpieces to be ground are, because in the case of longer workpieces, more bezel seats generally have to be ground. Self-centering bezels have the property that the workpiece is clamped so as to be centered in the three bezel jaws. This clamping during support has the disadvantage that optical running tracks are retained at the support location of the bezels, on the finished, ground, rotationally symmetrical section.
An additional problem in grinding between tips arises from the fact that when setting multiple bezels, these possess a different thermal growth, in each instance, even if this lies only in the μm range. Because of this different thermal growth, imprecisions can be additionally introduced into the grinding process.